Electron cyclotron heating at down-shifted frequencies in existing tokamak devices

Plasma heating in existing tokamak devices by electron cyclotron waves with frequency (f) significantly smaller than the electron gyrofrequency (fsub(c)) is investigated for the case of Maxwellian plasmas. It is shown that for central electron temperatures larger than 3 keV, strong absorption of extraordinary waves can occur at values of the toroidal field for which the condition f=fsub(c) is not satisfied in the plasma region. The cases of f=60 GHz and f=100 GHz are discussed for the PLT and TFTR devices as being representative of medium (approx. 30 kG) and high (approx. 50 kG) magnetic field tokamaks, respectively. Numerical calculations with a ray tracing code indicate that most of the RF energy is absorbed in a central plasma region. These results are of practical interest because they significantly simplify the main technical problem of ECH in a tokamak reactor, i.e. the development of high-frequency and high-power microwave sources. (author)